Method of manufacturing thin compound oxide film and apparatus for manufacturing thin oxide film

ABSTRACT

In a method of manufacturing a thin film of a compound oxide, different types of materials for forming the compound oxide are evaporated in vacuum. The evaporated materials are heated and deposited on a substrate to form a thin film. An oxygen ion beam having energy of 10 to 200 eV is implanted in the thin film which is being formed on the substrate. Alignment of the constituting elements is performed on the basis of a substrate temperature and energy of the oxygen ion beam, thereby causing epitaxial growth.

BACKGROUND OF THE INVENTION

The present invention relates to a method of manufacturing a thincompound oxide film used in a light modulation element, a piezoelectricelement, a surface acoustic wave element, an oxide superconductorelement, an LSI, and an EL and an apparatus for manufacturing a thinoxide film.

A known conventional method of manufacturing a thin compound oxide filmis a sputtering method. In this method, epitaxial growth of the thinfilm can be performed to prepare a single-crystal composite oxide suchas PZT[Pb(ZrTi)O₃ ], PLZT[(PbLa)(ZrTi)O₃ ], PbTiO₃, and BaPb_(1-x)Bi_(x) O₃.

Successful examples of the methods of sputtering thin compound oxidefilms are limited to those for a limited number of oxides describedabove. Since control of the oxygen content depends on an oxygen partialpressure, control on the atomic level cannot be accomplished. Goodreproducibility cannot be assured due to change in a target compositionratio with time. Thus, oxygen defects are formed in the resultant thinfilm, density of the film is lowered, or composition errors occur.Therefore, thin films required in other various fields of applicationscannot be obtained. In addition, according to a conventional sputteringmethod, a gas is adsorbed on the surface of the substrate. In order toepitaxially grow atomic particles bombarding the surface of thesubstrate, atoms deposited on the substrate must be moved to a propercrystal site and must be aligned. However, such movement of atoms isprevented by the adsorbed gas. In the case of an oxide film, oxygenatoms for forming an oxide in addition to the adsorbed gas preventmovement of the deposited atoms. For these reasons, the substrate mustbe heated to a temperature of 500° C. or higher. As a result, filmconstituting elements are undesirably diffused into the under-layers ifthey are multilayered films having different materials.

Another conventional technique for forming a thin film is to deposite amaterial onto a substrate and at the same time to implant oxgen ionbeams, as described in "ion-beam-assisted deposition of thin films"(APPLIED OPTICS, Vol. 22, No. 1, Jan. 1, 1983, P. J. Martin et al.) and"Review ion-based methods for optical thin film deposition" (JOURNAL OFMATERIALS SCIENCE 21 (1986) PP. 1-25, P. J. Martin).

The former literature deals with a thin film of an oxide of a simplesubstance such as TiO₂, unlike in the thin film of a compound oxideaccording to the present invention. In addition, energy of an ion beamis as high as 600 to 750 eV, and the resultant thin film tends to beamorphous. In the latter literature, since an operation gas pressure ofan ion source is 1×10⁻⁴ Torr or more, no deposition is performed in ahigh vacuum. Deposition molecules of the material repeatedly collidewith a residual gas and reach the substrate. Therefore, no controlledmolecular beam can be obtained. In addition, adsorption occurs on thesubstrate surface due to a residual gas. Therefore, an initiationtemperature of epitaxial growth is undesirably increased. Energy of theoxygen ion beam incident on the substrate adversely affectscrystallinity of a thin film to be formed. Selective control of theenergy range for preventing crystal defects cannot be performed. Theimplanted oxygen beam includes O⁺, O₂ ⁺, O₂ ⁺⁺, and O (neutral). Theaccurate number of oxygen atoms incident on the substrate cannottherefore be controlled.

SUMMARY OF THE INVENTION

The present invention has an object to eliminate the conventionaldrawbacks described above, and has as its object to provide a method ofmanufacturing a thin film of a compound oxide wherein energy of anoxygen beam can be optimized, precise composition control can beachieved, a high-density thin oxide film can be formed, andlow-temperature epitaxial growth can be achieved by a combination of anenergy effect of the oxygen beam and a high vacuum, and an apparatus forforming a thin oxide film.

In order to achieve the above object of the present invention, there isprovided a method of manufacturing a thin compound oxide film,comprising the steps of: evaporating different types of elements forforming a compound oxide; depositing the different types of evaporatedelements on a substrate and moving the deposited elements to a propercrystal site; and implanting oxygen ions or neutral oxygen particleshaving energy of 10 to 200 eV in a growing thin film, and aligning theelements and the oxygen ions or neutral oxygen particles as a crystalfilm of a compound oxide, thus epitaxially growing the thin compoundoxide film.

An apparatus for forming a thin oxide film comprises a film formationchamber which is adapted to be evacuated and in which a substrate isplaced; material feed means for feeding a material of a thin oxide filmto be formed on the substrate; oxygen ion generating means; massseparating means for separating oxygen ions from an oxygen plasmagenerated by the oxygen ion generating means; and oxygen iondecelerating means for decelerating the separated oxygen ions to a speedcorresponding to energy of 10 to 200 eV and for implanting thedecelerated oxygen ions as an oxygen ion beam in a thin oxide film whichis being formed on the substrate.

With the above constitution of the present invention, the method ofmanufacturing a thin compound oxide film and an apparatus formanufacturing a thin oxide film provide a thin film free from oxygendefects and composition errors. Therefore, a thin film has a densitysimilar to a bulk value, and a substrate temperature can be reduced.Therefore, the crystallization initiation temperature can be reduced andlow-temperature epitaxial growth is possible, wherein thin-filmconstituting elements diffuse in the direction of thickness of the thinfilm. According to the present invention, energy and ion beam density ofthe oxygen ion beam can fall within the optimal ranges, and thus precisecomposition control can be performed. Electrical and opticalcharacteristics of the thin oxide film widely used in elements such as alight modulation element, a piezoelectric element, a surface acousticwave element, a superconductor element, an LSI, and an EL can be greatlyimproved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing an arrangement of an apparatus formanufacturing a thin oxide film for realizing a method of forming a thincompound oxide film according to an embodiment of the present invention;

FIG. 2 is a graph showing the maximum oxygen ion current density as afunction of oxygen ion energy generated by the apparatus shown in FIG.1;

FIG. 3 is a side view of a thin compound oxide film formed on asubstrate by the method of the present invention and the apparatustherefor;

FIG. 4 is a side view of a thin oxide film formed on a substrate througha buffer layer by the method of the present invention and the apparatustherefor according to another embodiment; and

FIG. 5 is a side view of a thin oxide film having a multilayeredstructure formed by the method and the apparatus therefor according tostill another embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A method of manufacturing a thin compound oxide film according to anembodiment of the present invention will be described with reference tothe accompanying drawings. Metal materials (e.g., Nb and Li) for forminga thin compound oxide film are evaporated in a vacuum pressure of theorder of 10⁻⁹ Torr or less (i.e., vacuum pressure of the order of 10⁻¹⁰Torr, 10⁻¹¹ Torr, etc.) because the substrate need not be heated to 500°C. or higher so that the adsorbed gas on the substrate surface does notprevent free movement of material particles bombarding on the substrateto a predetermined site thereon. A means for evaporating metal materialsmay comprise a resistive heater, an electron gun assembly, or a laserand is not limited to any specific type.

The substrate preferably comprises a single-crystal substrate having thesame lattice constant as that of the thin film. If a substrate having adesired lattice constant cannot be obtained, a buffer layer having as alattice constant an intermediate value between the lattice constant ofthe thin compound oxide film and the actual lattice constant of thesubstrate is formed on the substrate, thereby absorbing a latticeconstant difference. Thereafter, a thin compound oxide film is formed onthe buffer layer.

A heating temperature of the substrate is preferably about 400° C. orlower. When a thin film of a multilayered structure is formed at atemperature exceeding 400° C. or when a buffer layer is formed on thesubstrate and a thin compound oxide film is formed thereon at atemperature exceeding 400° C., diffusion of the material elements occursin the direction of the multilayered thin film or the buffer layer. As aresult, the properties of the thin film may be impaired. The lower limitof the temperature is practically about 200° C. The evaporated metalmaterials are deposited on the substrate. Oxygen ions O⁺ or O₂ + aredoped in the thin film which is being formed on the substrate. Theseions are generated by an ion source and the oxygen ions O⁺ or O₂ ⁺ areseparated by a mass separator. The separated ions are decelerated by anion deceleration electrode to a speed corresponding to energy of 10 to200 eV. The decelerated oxygen ions are implanted in the thin film. Ifion energy is less than 10 eV, energy for epitaxially growing the oxidefilm is insufficient. However, if ion energy exceeds 200 eV, thecrystallized thin film may be damaged.

An apparatus for manufacturing a thin compound oxide film formed asdescribed above according to the present invention will be describedwith reference to FIG. 1.

Substrate 3 held in holder 2 is placed in film formation chamber 11shown in FIG. 1. Substrate 3 is heated by heater 18 to about 400° C.Heating of substrate 3 is performed as needed. K-cells 8 and 8" servingas molecular beam sources and electron beam vapor source 9 are locatedat positions opposite to substrate 3. K-cells 8 and 8" and vapor source9 are used to evaporate different thin film materials, respectively.Film formation chamber 11 can be evacuated by a cryopump or the like tovacuum pressure of the order of 10⁻⁹ Torr or less. Ionizing chamber 10comprises a plasma source (not shown) therein to generate an oxygenplasma. The generated ions O⁺ or O₂ ⁺ pass through mass separator 13 andoxygen ions O⁺ or O₂ ⁺ are separated thereby. Ionizing chamber 10 andfilm formation chamber 11 are separated from each other by gate valve 20and can be independently evacuated. Oxygen ions O⁺ or O₂ ⁺ having highenergy of 10 to 50 keV are decelerated to have energy of 10 to 200 eV byion deceleration electrode 12. The decelerated oxygen ions O⁺ or O₂ ⁺are neutralized by electrons emitted from electron emission source 17made of a ring-like thermionic filament. Oxygen atoms or molecules O orO₂ are implanted at a constant speed in the thin film which is beingformed on the substrate. When an insulating film is to be formed on thesubstrate, charging of the film can be prevented. In the illustratedembodiment, electron emission source 17 is arranged. However, this maybe omitted. In this case, oxygen ions O⁺ or O₂ ⁺ may be doped in thethin film.

Ion gauge 14 (or a quartz oscillator) and mass analyzer 15 are locatedat predetermined positions inside film formation chamber 11. Intensitiesof molecular or ion beams emitted from K-cells 8 and 8' serving as themolecular beam sources or electron beam vapor source 9 are monitored,and monitor signals are fed back to control vapor amounts of the thinfilm materials.

The oxygen ion beam decelerated by deceleration electrode 12 ismonitored by Faraday cup 16 arranged inside film formation chamber 11.Therefore, a current density (corresponding to the number of oxygen ionsincident on the film) of oxygen ions incident on the thin film onsubstrate 3 can be detected by monitor signals. The current density isabout 10 to 100 μA/cm². Under the above-described control, the thincompound oxide film formed on substrate 2 is subjected to high-precisioncomposition control of constituting elements as well as high-precisioncontrol of energy and current density of the oxygen ion beam.

A method of manufacturing a thin film of a compound oxide will bedescribed in more detail.

A microwave excitation ion source was used as ionizing chamber 10 shownin FIG. 1. Even if a strongly reactive oxygen gas was used, filmformation continued for 10 hours or longer without maintenance of theion source. The degree of vacuum in ionizing chamber 10 during operationwas 5×10⁻⁴ Torr. A portion between ionizing source 10 and mass separator13 was evacuated by a cryopump (having a delivery rate of 700 l/sec) tomaintain the degree of vacuum pressure of film formation chamber 11 tobe 5×10⁻⁹ Torr.

Ions were extracted from ionizing chamber 10 at an extraction voltage of20 kV, and ions O⁺ were separated by mass separator 13. Ions O⁺ weredecelerated by decelerating electrode 12 to a speed corresponding toenergy of 10 to 200 eV. In this case, the oxygen ion current density asa function of energy after deceleration is shown in FIG. 2. As isapparent from FIG. 2, a maximum ion current density of 50 μA/cm² can beobtained at energy of, e.g., 50 eV.

EXAMPLE 1 Fabrication of Thin Film of Single Crystal LiNbO₃

C-plate sapphire (lattice constant aH=4.758 Å) was placed as substrate 3on substrate holder 2 arranged inside film formation chamber 1 shown inFIG. 1.

The vacuum pressure was set to the order of 10⁻⁹ Torr or less, andsubstrate 3 was heated by heater 18 to a temperature of 350° C. Gatevalve 20 was opened, and oxygen ions O⁺ or O₂ ⁺ generated by ionizingchamber 10 on the basis of plasma energy were accelerated and output.Only O⁺ ions were separated by mass separator 13. The ions O⁺ weredecelerated to a speed corresponding to energy of about 100 eV through aslit of ion deceleration electrode 12. Oxygen ions O⁺ were caused topass through a thermion shower as electrons emitted from electronemission electrode 17. Therefore, neutral 0 atoms were bombarded onsubstrate 3.

Li was heated and evaporated from K-cells 8 and 8" serving as molecularbeam sources and Nb was evaporated by electron beam vapor source 9 todeposit Li and Nb on substrate 3. Meanwhile, 0 atoms were doped in thethin film so that an LiNbO₃ thin film was formed. The cross section ofthe thin film of the compound oxide prepared as described above is shownin FIG. 3.

As is apparent from FIG. 3, LiNbO₃ thin film 30 is deposited onsubstrate 3.

When a deposition rate of LiNbO₃ is 3,000 Å/hour and the thickness ofthe film was 3,000 Å to 1 μm, refractive index n_(o) (ordinary light)and refractive index n_(e) (extraordinary light) were respectively 2.32and 2.18 for light having a wavelength of 6.33 Å. In this case, anelectrooptical coefficient of the film was 29.3, which was close to abulk coefficient.

It is assumed that the above results are obtained by oxygen implantingsince the density of the deposited film is near a bulk density.

The LiNbO₃ thin film prepared in Example 1 had a light loss of 10 dB/cm.

EXAMPLE 2 Fabrication of LiNbO₃ Thin Film On Substrate with Buffer Layer

Prior to the steps in Example 1, a 100 to 300 Å thick Li(Nb, Ti)O_(x)film having an intermediate lattice constant was formed on substrate 3to absorb a difference between a lattice constant (4.75 Å) of sapphiresubstrate 3 and a lattice constant (5.14 Å) of LiNbO₃ and make epitaxialgrowth easier.

Following the same procedures as in Example 1, a thin film of an LiNbO₃compound oxide was formed.

The cross section of the resultant film is shown in FIG. 4. In Example2, buffer layer 31 is formed on substrate 3, and LiNbO₃ thin film 30 isformed on buffer layer 31.

Refractive indices n_(o) and n_(e) of the resultant thin film wererespectively 2.29 and 2.2 for light having a wavelength of 6.328 Å, andan electrooptical coefficient (r33) thereof was 31.4, which were closeto those of single crystal (bulk). These results are assumed to beobtained by the pressure of buffer layer 31 and an effect of oxygenimplantation. The sample was too small to measure the light loss.

EXAMPLE 3 Fabrication of BaPbO₃ -BaBiO₃ Multilayered Film

Metal particles or powders, i.e., Ba, Pb, and Bi particles or powderswere put into K-cells 8 and 8' and an additional K-cell (not shown)placed in film formation layer 1 shown in FIG. 1, respectively, and anSrTiO₃ (110 plane) single-crystal substrate was used as substrate 3.

The vacuum pressure was set to the order of 10⁻⁹ Torr or less, and asubstrate temperature was set to 350° C. A first layer was formed onsubstrate 3, as indicated by the cross section of FIG. 5. Morespecifically, Ba and Bi were respetively evaporated from K-cells 8 and8'. Oxygen ions O⁺ or oxygen atoms O were doped in a BaBi film while Baand Bi were being deposited on substrate 3, thereby forming BaBiO₃ layer32. In this case, energy of oxygen ions O⁺ or O was 100 eV.

The shutter of the additional K-cell was closed to stop evaporating Bi.Ba was continuously evaporated. In addition, Pb was evaporated. O⁺ or Owas doped in a BaPb film while Ba and Pb were being deposited onsubstrate 3, thereby forming BaPbO₃ layer 33.

By repeating the above steps, 30 BaBiO₃ layers 32 each having athickness of 30 to 100 Å and 30 BaPbO₃ layers 33 were alternately formedto prepare a multilayered film.

Each thin film was a single-crystal film prepared by epitaxial growth.As a result of an X-ray diffraction test, a diffusion layer between eachBaBiO₃ layer and the BaPbO₃ layer adjacent thereto had a thickness of 10Å or less.

In Example 3, the multilayered film is epitaxially grown on asingle-crystal substrate. However, when a polycrytalline film was formedon, e.g., a glass substrate, crystallization began at a low substratetemperature according to experiments. In Examples 1 to 3, the method ofmanufacturing compound oxides has been exemplified. However, thin oxidefilms can be formed by the apparatus shown in FIG. 1, and this will bedescribed below.

Zn metal particles filled in K-cell 8 were evaporated, and a K-celltemperature was controlled such that a deposition rate on substrate 3was set to 0.73 Å/sec. At the same time, when an oxygen ion beam havingenergy of 50 eV and a current density of 50 μA/cm² was incident on theglass substrate, the number of Zn atoms incident per unit area and perunit time was equal to that of 0 atoms, thus obtaining a polycrystallineZnO film having a stoichiometric ratio Zn:0=1:1. In particular, R- orC-plane sapphire was used as a substrate and the substrate was kept at aconstant temperature falling within the range of room temperature to600° C. In this case, a ZnO single-crystal film having a smooth surfacecould be prepared. The degree of vacuum in film formation chamber 11during Zn evaporation was 5×10⁻⁸ Torr. A ZnO (1120) plane and a ZnO(0001) plane were respectively expitaxially grown on the sapphire R- andC-planes. In both cases, half-widths of the (1120) and (0001) peaks wereas small as 0.3 or less according to an X-ray diffraction test. The ZnOlattice constant converted based on the peak position was substantiallythe same as the bulk value (a difference was 0.1% or less), thuspresenting good crystallinity. In general, good crystallinity cannot beobtained by sputtering unless a substrate temperature is 600° C. orhigher. According to the apparatus for manufacturing the thin oxide filmaccording to the present invention, it is proved that a single-crystalfilm having good crystallinity can be prepared at a low substratetemperature.

What is claimed is:
 1. A method of manufacturing a thin single crystalfilm of a compound oxide, comprising the steps of:maintaining a vacuumpressure in the order of 10⁻⁹ Torr or less; heating a substrate to atemperature of less than 400° C.; evaporating different types ofmaterials for forming the compound oxide; depositing the evaporatedmaterials on a substrate to form the thin film; generating oxygen ions,the oxygen ion generating step including the steps of generating anoxygen plasma, mass-separating oxygen ions from the accelerated oxygenplasma to separate O⁺ or O₂ ⁺ ions, and decelerating the separatedoxygen ions to a speed corresponding to energy of 10 to 200 eV; andimplanting oxygen ions, decelerated in the decelerating step and havingenergy of 10 to 200 eV, in the thin film as it is being formed on saidsubstrate while monitoring and controlling the number of oxygen ions. 2.The method according to claim 1, further including the step ofneutralizing the O⁺ or O₂ ⁺, which emits electrons to obtain the neutraloxygen particles, after the decelerating step.
 3. The method accordingto claim 2, wherein the electron emitting step comprises the step ofradiating electrons by an electron shower.
 4. The method according toclaim 1, wherein the step of depositing the evaporated materials on asubstrate to form the thin film includes the step of forming a bufferlayer on the substrate, a lattice constant of the buffer layer being anintermediate value between a lattice constant of the substrate and alattice constant of the thin film.
 5. The method according to claim 1,wherein the substrate comprises a single crystal and is heated to atemperature of about 400° C. or less.
 6. The method according to claim1, wherein the material evaporating step, the thin film forming step,and the oxygen ion implanting step are repeated a plurality of times toform a multilayered thin film of the compound oxide.